This invention relates to fluids used in treating a subterranean formation. In particular, the invention relates to shear resistant crosslinked polymer fluids that contain a water-soluble polymer, a crosslinking agent, and an acidic pH adjusting agent, as well as methods of treating a subterranean formation with such fluids.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. In the recovery of hydrocarbons from subterranean formations it is common practice, particularly in low permeability formations, to fracture the hydrocarbon-bearing formation (i.e. to create a fracture or create a less resistance path for the formation fluids) to enhance oil and gas recovery. In such fracturing operations, a fracturing fluid that is capable of suspending a proppant is hydraulically injected into a wellbore that penetrates a subterranean formation. The fracturing fluid is forced against the formation strata by applying sufficient pressure to the extent that the fracturing fluid opens a fracture in the formation. This pressure is then maintained while injecting fracturing fluid at a sufficient rate to further extend the fracture in the formation. As the formation strata or rock is forced to crack and fracture, a proppant is placed in the fracture by movement of a viscous fluid containing proppant into the crack in the rock. After the pressure is reduced, the fracture closes on the proppant, thus preventing complete closure of the fracture. The resulting fracture, with proppant in place, provides improved flow of the recoverable fluid, i.e., oil, gas, or water, into the wellbore.
Natural polymers such as guar and its derivatives have been widely used as gelling agents to prepare viscous fracturing fluids for hydraulic fracturing treatments. These polymers are water soluble polymers. Examples of such water-soluble crosslinkable polymers include guar, hydroxypropyl guar (HPG), carboxymethyl guar (CMG), carboxymethyl hydroxypropyl guar (CMHPG), carboxymethyl cellulose (CMC), etc. For such treatments, the polymer is combined with an aqueous fluid so that the polymer is hydrated to give a linear gel solution and then crosslinked to give a viscous gel solution. Crosslinking may occur instantly or be delayed until the fluid reaches the desired location.
Various fluid systems employing guar and guar derivative polymers are used today that employ crosslinking agents such as zirconium compound crosslinkers. Some typical gelling agents for zirconium crosslinkers include HPG, CMG, and CMHPG. Zirconium based crosslinkers have found widespread application for fracturing fluids used in deep and high temperature reservoirs. Such zirconium crosslinked polymer fluids, however, are known to be shear-sensitive. In particular, high shear can cause the loss of fluid viscosity. The exact mechanism for this degradation is still a matter of debate and the oil and gas production industry has dealt with this shortcoming for many years.
Various methods for addressing the high shear sensitivity of these fluids have been used in the past. Primarily this involves delaying the onset of crosslinking of the fluid so that crosslinking is avoided or retarded in the high-shear (e.g. shear of 500/sec or more) regions, such as is encountered within the tubulars going from the surface to reservoir depth. Once the fluid enters the fracture or low-shear regions (generally 10 to 200/sec), the crosslinking is then initiated. Typical methods of delaying crosslinking include the use of complexing agents, for example bicarbonate or lactate, that allow a ligand exchange of the metal to polymer at an elevated temperatures.
Such delayed crosslinked fluids may be designed for use in high temperature or low temperature environments. In low temperature environments (e.g. less than 121° C.), the pH of the fluid may be lower. A lower starting pH, for example, about 5, contributes to better shear recovery, usually defined as viscosity recovery after shearing, of the fluid. At higher temperatures (e.g. 121° C. or greater) the low pH fluids will deteriorate the viscosity due to polymer instability in low pH and high temperature environments. As a result, in high temperature environments, a starting pH of, for example, above 9, is usually used so that the fluid does not become too acidic to damage the polymer. Such high pH fluids, however, may often exhibit poor shear recovery.
This invention describes the means of avoiding these limitations and providing the methods of generating fracturing fluids that do not suffer irreversible shear degradation.